(CNN) -- An Arizona judge on Wednesday refused to grant a new trial for self-help guru James Arthur Ray in the deaths of three participants in a 2009 sweat lodge ritual.

Ray was convicted of negligent homicide in June, but his attorneys argued that prosecutors failed to disclose evidence on time. Yavapai County Superior Court Judge Warren Darrow denied that motion Wednesday morning, and Ray faces up to 12 and a half years in prison at his September 26 sentencing.

Three people died and at least 15 others fell ill in October 2009 during a "Spiritual Warrior" retreat -- a ceremony modeled after Native American purification rituals -- that Ray held at a resort near Sedona. Prosecutors argued that the lodge, which was made of willow trees and branches and covered with tarpaulins and blankets, was heated to a perilously high temperature and that Ray was indifferent to those clearly having trouble.

Ray's lawyers countered that what happened was a tragic accident, not a crime. They also suggested that exposure to an unknown toxin in the lodge -- perhaps a pesticide, rat poison or something in the type of wood used to heat the rocks -- could have caused the deaths.